1. Field of the Invention
The present invention relates to an air conditioning system, and more particularly to a system circulating a phase-changeable fluid between gas and liquid phases between a heat source side machine and a plurality of user side machines all or more than half of which are disposed below the heat source side machine, so that each of the user side machines can perform cooling or heating operation.
2. Background Art
An example of the above-mentioned type air conditioning system is shown in FIG. 12. This air conditioning system includes a heat source side machine 1 that performs cooling or heating selectively, and user side machines 2 all or more than half of which are disposed below the heat source side machine 1. The heat source side machine 1 and each of the user side machines 2 are connected with each other via a liquid phase pipe 3 and a gas phase pipe 4 so as to form a closed circuit 5. This closed circuit 5 includes a phase-changeable fluid, i.e., a refrigerant. When this refrigerant is cooled and condensed in the heat source side machine 1, the condensed liquid refrigerant is led to each of the user side machines 2 by opening a cooling/heating switch valve 6 provided in the liquid phase pipe 3. Then, a heat exchanger 7 of each user side machine 2 performs heat exchange between the refrigerant and room air so as to perform cooling operation. During this heat exchange, the refrigerant gains heat to evaporate. The evaporated gas refrigerant flows in the gas phase pipe 4 and backs to the heat source side machine 1 that has been in a low pressure. Thus, cooling operation is performed in each of the user side machines 2.
When the refrigerant is heated and evaporated in the heat source side machine 1, the gas refrigerant evaporated in the heat source side machine 1 is led to each of the user side machines 2 via the gas phase pipe 4. Then, the heat exchanger 7 of each user side machine 2 performs heat exchange between the refrigerant and room air to perform heating operation. The liquid refrigerant condensed after discharging heat in the heat exchange process flows back to the heat source side machine 1 utilizing a discharging force of the electric pump 8 provided in the liquid phase pipe 3. Thus, each of the user side machines 2 performs heating operation.
Reference numeral 9 denotes a flow control valve, reference numeral 10 denotes a receiver tank, reference numeral 11 denotes a cooling/hating switch valve and reference numeral 12 denotes a blower.
In the above mentioned configuration of the air conditioning system, the phase-changeable refrigerant circulates in the closed circuit by its specific gravity difference between liquid and gas phases. Therefore, the configuration has an advantage in that power consumption can be reduced.
There is another configuration of the air conditioning system in which the liquid phase pipe 3 of the heat source side machine 1 is provided with a receiver tank 13 and an electric pump 14 that works in cooling operation as shown in the broken line in the figure. This configuration can enhance circulation ability of the refrigerant, so that some of the user side machines 2 can be placed at a little higher position than the heat source side machine 1. This electric pump 14 can be compact compared with the electric pump 8 that needs to have power for driving the liquid refrigerant condensed in the user side machine 2 to the heat source side machine 1 disposed at an upper place. Therefore, this configuration has an effect of reducing power consumption compared with the air conditioning system piped so that the circulation in the cooling operation can be performed utilizing the electric pump 8.
Another configuration of the system is disclosed in Unexamined Japanese Patent Publication Hei 7-151359 for example, in which the liquid phase pipe is provided with four open-close valves disposed closely to the electric pump 8 so as to use the electric pump 8 also for forcing circulation of the refrigerant in the cooling operation.
However, in each of the above mentioned configuration of the air conditioning system performing cooling operation, pressure of the phase changeable refrigerant in the closed circuit is always altering due to load variation or other factors. Thus, the liquid refrigerant can generate bubbles by evaporating partially when the pressure drops.
Especially when starting the cooling operation, the temperature of the liquid refrigerant is relatively high being heated by outside air even if the liquid phase pipe is covered with a heat insulator. Therefore, the refrigerant in the liquid phase pipe may bubble in concert if the pressure of the closed circuit drops rapidly after start of the cooling operation in the heat source side machine. If a cooling load is small and thus the amount of the circulating refrigerant is little, the refrigerant is easily affected by the outside air. In this partial load operation, the refrigerant may bubble in the liquid phase pipe responding to a slight pressure drop. Furthermore, the bubbles can be generated by the outside air invading into devices provided to the pipe.
The bubbles generated in the liquid phase pipe as mentioned above can make circulation of the liquid refrigerant unstable or distribution of the liquid refrigerant to each user side machine uncertain. As a result, it may happen that a room cannot be cooled sufficiently. Furthermore, since the amount of the refrigerant increases on the surface, it becomes difficult to circulate the refrigerant to each user side machine, and the cooling operation may be difficult to continue.
Furthermore, in the above mentioned air conditioning system, the liquid refrigerant condensed after discharging heat in the heat source side machine enters each of the user side machines and evaporates. Then the refrigerant flows back to the heat source side machine. Thus, a one-way path is formed. Therefore, there is a disadvantage in that it is difficult to remove bubbles if generated, and the bad affection of the bubbles lasts long hours. In this case, the bubbles gathered in an inlet of each user side machine should be removed from an outlet by once fully opening an expansion valve. However, this operation can cause a bad circulation of the refrigerant since not only bubbles but also liquid refrigerant can be removed from the outlet of the user side machine, resulting in detention of the liquid refrigerant in the gas phase pipe (so-called liquid back).
Therefore, it is required to remove the bubbles quickly if the refrigerant flowing in the liquid phase pipe generates bubbles during cooling operation.
On the other hand, during heating operation, the gas refrigerant heated and evaporated in the heat source side machine can be cooled and condensed in the gas phase pipe. Especially, when starting the heating operation, the gas pipe is substantially at a low temperature even if it is covered with an insulator. Therefore, the gas refrigerant heated and evaporated in the heat source side machine is easily condensed in the gas phase pipe. The condensed refrigerant generated in the gas phase pipe may cause unstable circulation of the refrigerant to the user side machine or uncertain distribution of the refrigerant to each of the user side machine, resulting in a problem of insufficient heating of a room. In addition, if the refrigerant is condensed and remains in the pipe, the refrigerant becomes insufficient on the surface and the operation may stop.
Therefore, if the refrigerant flowing in the gas phase pipe is condensed in the heating operation, it is necessary to remove the liquid refrigerant in the pipe quickly.